lets imagine a car on top of a shelter.

in a bad situation debris could hit vehicle and cause a fuel leak.

would you want to be in a enclosed space with gasoline leaking in and
no easy way to get out? fumes might overcome you...

someone menioned cellphones, in a emergency forget them.

the cell providers can and will by the push of a button prohibit all
but emergency workers calls. its programmed in the day you get your
phone. so the system doesnt overload and crash...

plus cell sites depoend on powerlines and if that fails battery backup.
battery power is only good for a hour at most sites.

on landbased phones backups are good for a week, by federal law.

ideally you have a shelter with 2 access points one indoors one
outdoors, and some minimal supplies, water, high calorie long life
food, like MREs meals ready to eat, and basic tools. flashlights,
battreries, radio batter powered.

alowc some extra space, your neighbors will want protection too